It is already known to provide multilayer extrusion heads in which two or more layers or walls are extruded in a structure by passing the flow of one synthetic resin material through one passage and causing another synthetic resin material to flow codirectionally with the first flow, around the latter so as to meet it at a point in which the flows join to provide a multilayer structure. One such device is illustrated and described in U.S. Pat. No. 4,364,882. In that device, the flow from an extruder is passed axially and split into two generally annular streams which continue along separate paths until they meet a codirectionally moving stream from a second extruder which is forced into an annular passage between the first two streams.
When the streams meet, they form a triple-wall coextrusion which can be condensed in thickness, if desired, to emerge from the extrusion head as a tubular structure having inner and outer layers which derive from the synthetic resin material of the first extruder, and an intermediate layer sandwiched and bonded to the outer and inner layers, which derives from the second extruder.
Problems are encountered with such systems in part because the passage communicating with the intermediate annular passage is at right angles to the latter and to the axis of the extrusion head.
When a right-angle flow from a radial passage, for example into an annular passage is effected, the flow distribution tends to be irregular and such irregularities can result in a defect in the operation and in the product made.